Fix cross-endpoint querying so they can be combined

[flaneuse]

Currently, if there are multiple cross-endpoint queries, only the second one gets executed. Requires rewriting the query parser to separate and combine queries. At least at first, queries will probably all be AND'd.

Returns 1832 results as of 2 December 2019:
https://data.cvisb.org/api/patient/query?q=__all__&elisa=[[elisa.virus.keyword:Lassa AND elisa.assayType.keyword:Ag AND elisa.ELISAresult.keyword:negative AND elisa.timepoint.keyword:"patient admission"]]

Returns 369 results as of 2 December 2019
https://data.cvisb.org/api/patient/query?q=__all__&experimentQuery=includedInDataset:hla

ERROR: combined query only returns second query:
https://data.cvisb.org/api/patient/query?q=__all__&elisa=[[elisa.virus.keyword:Lassa%20AND%20elisa.assayType.keyword:Ag%20AND%20elisa.ELISAresult.keyword:negative%20AND%20elisa.timepoint.keyword:%22patient%20admission%22]]&experimentQuery=includedInDataset:hla